Very little can explicitly prevent a GM from inconsistent ad-hoc decisions. Rules can help shape the GM's decision space (eg: the presence of random tables by location for encounters, and random encounter procedures based on a x in 6 chance etc); signal to the GM what's expected of them and provide heuristics on decision making (Agendas and Principles in a PBTA, GM guidelines in OSE, etc); create gameplay that foregrounds stakes and player ideations (eg: FITD Action Rolls); and other similar design space.
The more open the GM decision making space is, the more open it is to unintentional inconsistency. This is why most games with fairly relaxed/loose mechanics that I've seen stress impartiality, preparation that fits within the procedures of the game (eg: Blorb style stuff), and recording rulings in an open way that creates a procedure from that moment forth.
Without any of that, the only thing preventing inconsistency is hope and wishes. Then you get sad posts to r/OSR or DMAcademy or whatever going "hey, so I did X in my game and I think it was wrong

."